The invention relates generally to the field of digital image processing and, more particularly, to a method for enlarging the size of a digital image with interpolation and coordinating the edge shaping of the resulting digital image with the characteristics of the interpolation.
It is common in the field of digital image processing to increase the resolution of a digital image. For example, suppose that it is desired that a digital image having resolution of 480 by 640 pixels be printed at a size of 3.84 inches by 5.12 inches on a device that prints only 250 dpi (dots per inch.) In this case, if the resolution of the image is exactly doubled, then the final print will be the correct size.
The process of changing the resolution of a digital image (especially when increasing the resolution) is referred to as interpolation. Interpolation is the process of estimating the value of a digital image signal at locations intermediate to the original samples.
The process of digital image interpolation is often performed with bilinear interpolation or bicubic interpolation. Both of these methods do indeed allow for the creation of high resolution digital images from low resolution digital images. However, the resulting high resolution digital image is inferior in quality to an image of the same scene which was originally captured with high resolution. The interpolated image appears to lack detail, the edges appear to be soft, and a jagged artifact due to edge aliasing may occur.
One method of increasing the apparent amount of detail and edge contrast in a digital image is by performing a sharpening operation. Traditional methods of increasing the apparent sharpness of a digital image, such as the technique of unsharp masking, often produce unwanted artifacts at large transition edges in the image. For example, unsharp masking is often described by the equation:
xe2x80x83Sproc=Sorg+B(Sorgxe2x88x92Sus)
where Sproc represents the processed image signal in which the high frequency components have been amplified, Sorg represents the original image signal, Sus represents the unsharp image signal, typically a smoothed image signal obtained by filtering the original image, and B represents the high frequency emphasis coefficient.
The unsharp masking operation may be modeled as a linear system. Thus, the magnitude of any frequency in Sproc is directly dependent upon the magnitude of that frequency in the Sorg image signal. As a consequence of this superposition principle, large edges in the Sorg image signal will often display a ringing artifact in the Sproc signal when the desired level of high frequency enhancement has been performed in other areas of the Sproc signal. This ringing artifact appears as a light or dark outline around the large edge, and may be visually objectionable.
Additionally, the optimum parameter values used for the sharpening stage are not independent of the method or amount of interpolation performed on the digital image. Parameter values found to produce good results for an interpolation factor of 2 may yield sub-optimal results for an interpolation factor of 8.
Thus, there exists a need for an alternative method of manipulating a digital image that has undergone an interpolation process in order to generate an image signal that appears to be sharper, or more in focus and while minimizing the ringing artifact commonly associated with unsharp masking. Additionally, there exists a need to automatically estimate the optimal parameters for the sharpening filter dependent upon the parameters of the interpolation algorithm.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention comprises a method for enhancing a digital image channel, the method comprising steps of:
(a) providing a digital image channel having image pixels;
(b) providing interpolation parameter values;
(c) generating an interpolated digital image channel using the interpolation parameter values and the digital image channel;
(d) generating edge shaper parameter values from the interpolation parameter values; and
(e) generating an enhanced digital image channel from the interpolated digital channel by applying an edge shaping algorithm controlled by the edge shaper parameter values.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
The present invention has the advantage of controlling the value of the central pixel of a digital image channel in accordance with a statistical characteristic of the region, e.g., driving the value of the central pixel either toward the local maximum or the local minimum of the region, except in the case where the central pixel is substantially midway between the local maximum and the local minimum. Consequently, edge transitions occur over a narrower range of pixels than in the input image, thus generating an image signal that appears to be sharper, or more in focus, than the original image. Moreover, since the output of the tone scale conversion is modified by the statistical characteristic, e.g., bounded by the local maximum and the local minimum of the region, systematic overshoot and undershoot at an edge boundary is diminished and the ringing artifact is not as noticeable.